Hysteresis in transport critical-current measurements of oxide superconductors

We have investigated magnetic hysteresis in transport critical-current (I-c ) measurements of Ag-matrix (Bi,Pb)(2)Sr2Ca2Cu3O10-x(Bi-2223) and AgMg-matr ix Bi2SrxCaCu2O8+x (Bi-2212) tapes. The effect of magnetic hysteresis on th e measured critical current of high temperature superconductors is a very i mportant consideration for every measurement procedure that involves more t han one sweep of magnetic field, changes in field angle, or changes in temp erature at a given field. The existence of this hysteresis is well known; h owever, the implications for a measurement standard or interlaboratory comp arisons are often ignored and the measurements are often made in the most e xpedient way. A key finding is that I-c at a given angle, determined by swe eping the angles in a given magnetic field, can be 17% different from the I -c determined after the angle was fixed in zero field and the magnet then r amped to the given field. Which value is correct is addressed in the contex t that the proper sequence of measurement conditions reflects the applicati on conditions. The hysteresis in angle-sweep and temperature-sweep data is related to the hysteresis observed when the field is swept up and down at c onstant angle and temperature. The necessity of heating a specimen to near its transition temperature to reset it to an initial state between measurem ents at different angles and temperatures is discussed.